(Reuters) - McDonald's popular $1 McDouble cheeseburger, which has lured customers to the Golden Arches since 2008, is getting hard to sustain as rising beef prices threaten the company's profit margin.

The world's biggest restaurant chain launched a competing $1 Grilled Onion Cheddar Burger in December. It also made the new sandwich - with one beef patty rather than the McDouble's two - a star of current television commercials, a status it shares with the McDouble.

The dilemma for McDonald's Corp (MCD.N) restaurant operators is that the McDouble has the highest ingredient costs on the Dollar Menu, making it a bad financial proposition unless customers add high-margin sides such as french fries or soda.

"If the McDouble is all the customer buys, you lose money," said Richard Adams, a former franchisee who now advises the chain's restaurant operators. "Depending on what happens to beef prices, McDonald's management should be open to taking the McDouble off the Dollar Menu."

The decision would be a significant one. McDonald's gets 10 to 15 percent of its sales from the Dollar Menu and experts say the McDouble is one of the most popular items on it.

Many franchisees, who pay royalties to the parent company based on overall sales, have exercised their option to move the McDouble off the Dollar Menu by raising its price over $1.

It's not just beef prices rising, it's everything that depends on corn. Going to see rising food costs across the board this year all because we use corn for fuel.

This is true. Here in the US, we had a horrible drought this past summer which ruined crops and forced farmers to sell off livestock.So we all knew here that prices of meat and produce would go through the roof.

the ratio of soy to beef is just going to get higher. I hate nothing more than when I go to buy just a regular box of burgers at the store, see it says "Beef patties", and only to find out when I get home that yea its beef, as well as half soy. All the people who claim you cant tell the difference when soy or turkey is substituted for beef are idiots. Now I either buy the ground beef myself and make the patties or only buy frozen ones that say specifically on the box "100% beef", and list only beef under its ingredients.